If you have concrete walls and cannot cover them with acoustic materials, at least treat the 1st reflections on all planes. In general, the back wall of the front speakers should be absorptive so that depth can be enhanced. Using a candle on top of your speaker as a visible light source, have somebody slide a mirror along your walls and ceiling and floor and determine your 1st refection points. the point where you see the candle is the 1st reflection point for that surface. You should have materials to break these reflections on your floor , ceiling and side walls as well as the back wall. with the rear wall, Place anything that can break reflections such as bookcases or anything with iregular shapes to direct the sound into different directions(diffusion). this is your first step into hearing what your system can do. Of course before doing this, your speakers should be away from the back and side walls.
